Heads-up display

In technology a heads-up display is a set of data that appears before a viewer without obstructing view. In the Doom games it consists of the status bar on the bottom of the screen and text that appears in the upper left corner of the screen, normally consisting of one-line text message in red lettering.

A message is normally displayed in the following circumstances:
 * An item has been picked up.
 * A key is needed to open a door.
 * In multiplayer mode, a player has sent a message, or left the game.
 * A game mode (messages, detail, gamma) has been changed.
 * A cheat code has been activated.

Messages are displayed for approximately four seconds and then disappear. The "Enter" key will re-display the previous message. The "F8" key will disable game-generated messages (and subsequently reenable them). ("F8" does not affect messages from other players.)

In multiplayer mode, there is a second line in the heads-up display that is used to compose messages to be sent to the other players. The "T" key begins message composition, and a horizontal caret appears. A message may then be typed, and sent with the "Enter" key, or one of several predefined messages (known as "chat macros" or "chat strings") may be sent by holding "Alt" and pressing the assigned number key. If there are more than two players, a message may be sent to a single specific player by using the first letter of that player's color instead of "T" ("G" for green, "I" for indigo, "B" for brown and "R" for red).

Many source ports have made enhancements to the heads-up display, such as scrolling message lines, additional game-generated messages, varying color and position of text, and modes that replace the status bar at the bottom with heads-up display data (commonly known as HUD mode).

Technical notes

 * The heads-up display code is also used for text messages in menu mode and in automap mode (including the level name).
 * Different messages are displayed when a key is needed for a local door versus a remote door:
 * "You need a color key to open this door"
 * "You need a color key to activate this object"
 * If the player picks up a key that they already have, then the pickup message won't display.
 * If the player uses the -turbo parameter with a value above 100, every so often a message will display to notify about it.

Development
Doom's HUD went through several stages during development. Beginning with Doom v0.2, the earliest concept for a HUD appears in the form of a complex view from inside the Doom marine's helmet. This view included (non-functional) health, armor, and shield meters; ammunition read-outs for each weapon; a display of statistics for the current weapon in use; a player inventory listing; an on-screen automap; and an area for text messages or dialogue. This screen was based on a pencil sketch by Adrian Carmack, and was created in response to Tom Hall's ideas as outlined in the Doom Bible.

In the following Doom v0.3 build, this HUD is retained with some enhancements (the automap component is now functional and updates when the player moves into a new blockmap cell). A low-profile mode is also added, which marks the first appearance of a status bar, though it is not yet functional to any degree.

In Doom v0.4 the helmet view was removed and replaced with a taller, redesigned status bar. This design was never fleshed out beyond its background, however. As of Doom v0.5, the final publicly available alpha, the status bar was again redesigned and became partially functional. However, it would yet again be redesigned, taking a form close to its final release design, by the Doom press release beta. In this version, the message widget was not yet an independent element of the HUD; instead, messages would be temporarily rendered over the status bar by temporarily replacing the right two-thirds of the bar with a blank concrete-textured background. A small yellow font was provided for this purpose.